6400-18 RGB Light Source & 5 Photosynthetic Assimilation

Summary non-photochemical processes (e.g. heat; ● The 6400-18 Red, Green, Blue (RGB) Blankenship, 2002). However, because other Light Source can deliver 2000 μmol m-2 wavelengths of light are absorbed and are s-1 white light†. equally capable of driving photosynthesis, it ● Three LED colors can be controlled is important to not only describe an absorp- independently to deliver any combina- tion spectrum, but also the photosynthetic tion desired. action spectrum for the absorbed light.

OTE OTE ● Absorption spectrum of a plant drives the effectiveness of each color’s photosyn- Action spectra describe the amount of CO2- thetic response. fixed or O2-released at a particular wave- ● LED light should be normalized to the length across the absorption spectrum for a absorption spectrum of the plant. leaf. As is the case for absorption spectra,

N N action spectra vary greatly between species Action spectrum of plants and RGB (McCree, 1972). For many C3 species, there is greater assimilation when plants are output illuminated with red light and to a lesser Not all incident radiation is absorbed by extent blue light than when illuminated with actively photosynthesizing leaves. Depending green light. on wavelength, light incident on a leaf may be reflected, transmitted through the leaf or The 6400-18 Red, Green, Blue (RGB) Light absorbed by the light harvesting complex to Source is a composite LED source comprised drive the light reactions. The proportion of of multiple diodes embedded within a tile. light absorbed at any particular wavelength The LED wavelength peaks of this commer- varies between and within species, and cially available tile are 460, 522 and 635 ± 5 frequently between individual leaves in a nm, corresponding to light in the blue, green plant canopy, dependent on canopy position and red regions of the light spectrum respec- and the history of the leaf. tively. Apparent white light is achieved by providing equal quanta of each LED color. Absorbed light drives the light reactions and This is not a broad-spectrum white such as variations in the amount of absorbed light solar radiation, but rather it is the composite alter the overall carbon assimilation rate. For of all three LED colors. The LED tile used in many higher plants, much of the photosyn- the RGB Light Source achieves white light† thetically active radiation (light with wave- levels of 2000 μmol m-2 s-1 with a spatial lengths between 400 – 700 nm) is absorbed uniformity of ± 10% over 90% of the output

PPLICATION PPLICATION with peaks near the absorption peaks of area. chlorophyll a and b (a: 430 and 660 nm; b: 455 and 645 nm). As a result, light with Plant response with RGB output wavelengths in the red and blue region of the

A

A Each LED color in the RGB Light Source can spectrum typically drive photosynthesis more be controlled independently. Due to this efficiently than other wavelengths. On a independence of control, one natural experi- quanta basis, red and blue often drive the ment is to examine the photosynthetic same amount of photosynthesis since the response to each LED color. Light responses extra energy in blue light is quenched by at each color were measured on 6 to 8 week

† White light is defined as equal quanta of red, green and blue light. old Arabidopsis thaliana plants using the 6400-17 was measured on single leaves from 6 to 8 week old Whole Plant Arabidopsis Chamber. A combination of plants. Chamber environmental conditions were as clay capping of the planting medium and slight over- noted above. There were no significant differences in pressure were used to suppress CO2 flux from the the photosynthetic response under different colored planting medium (see Application Note #4: Using the light (Figure 2A). The absorbed light was calculated Whole Plant Arabidopsis Chamber Effectively). Photo- from absorption spectra for Arabidopsis (see below). synthetic responses were measured at 600 μmol mol-1 The quantum yield was calculated for each color and CO2 and 50 – 70% relative humidity at approximately there was no difference in the rates between colors 25 °C. The plants were light acclimated under 600 (Figure 2B). μmol m-2 s-1 white light† until photosynthesis and transpiration were at steady state. Light responses A. were measured from high to low in a single color chosen at random. Plants were again acclimated to 600 μmol m-2 s-1 white light and the response was then repeated with a different color.

The photosynthetic responses to light for the different colored LEDs were similar (Figure 1). Based on the action spectrum discussed above, red light was ex- pected to have the greatest assimilation rate at each light intensity and green would produce lower assimi- lation rates. However, there was no difference between the photosynthetic responses to each of the colors, which was surprising based on previous studies (McCree, 1972).

B.

Figure 1: Light response of Arabidopsis rosette under different colored LEDs. The light intensity is the amount incident on the plane of the majority of leaves of the rosette. Values are the average photosynthetic Figure 2: Light response of Arabidopsis leaves under assimilation normalized to projected leaf area ± St.Err. different colored LEDs. A: The light intensity is the of 5 samples. absorbed light normalized to each LED output. B: The initial slopes of the light response curves. The quan- To eliminate any enhanced assimilation effect of tum yield was calculated from the linear relationship of greater light absorption by the overlapping leaves of absorbed light to assimilated CO (R2 > 0.95). The the rosette, photosynthetic responses to different colors 2 quantum yield for each LED color is reported in the of light were measured on individual leaves. The RGB upper right. Values are the average photosynthetic Light Source was placed over a standard clear-topped assimilation normalized to leaf area ± St.Err. of 4 2 × 3 cm chamber and the photosynthetic assimilation samples. 2 The leaf-level photosynthetic light response of field- grown soybean leaves was also investigated. Indi- vidual soybean (cv.U98-311442) leaves were mea- sured following the same protocol with a 2 × 3 stan- dard chamber except that [CO2] was controlled at 380 μmol mol-1 and light intensities were higher. The different light colors drove photosynthesis at similar assimilation rates (Figure 3).

Figure 4: Absorption spectrum for Arabidopsis and soybean leaves measured from the adaxial side. The white output spectrum derived from equal quanta of three types of LEDs shows the bandwidth of each color.

However, the LED output in combination with the Figure 3: Light response of a soybean leaf under absorption spectra show that light absorption was different colored LEDs. The absorbed LED light was similar for each LED color type. With similar light calculated using the absorption spectrum of the leaf absorption, the lack of a difference in the photosyn- and the spectral output of the each LED type or the thetic responses to color is clear. For both Arabidopsis combination of all three colors for white. and soybean, the red LED output (peak 635 nm) is not centered on the absorption peak (approximately 680 The similarity in photosynthetic responses at different nm). Additionally, there is substantial absorption of the colors of light for the two species suggested that the green LED as it overlaps into the blue and red. The absorption should be similar. The absorption spectra calculated weighted absorption (α ) is the product of of Arabidopsis and soybean were measured from 400 w the leaf absorption (α) and the LED quantum output to 700 nm in 1 nm increments using a spectroradio- (Q) from wavelengths (λ) 400 to 700 nm and is nor- meter (LI-1800 Portable Spectroradiometer, LI-COR malized to Q. Biosciences, Lincoln, NE) and integrating sphere

(1800-12S External Integrating Sphere, LI-COR Bio- 700 sciences). Arabidopsis absorption was greatest in the αλ( )Qd( λ) λ ∫400 blue and red wavelengths and was 30% lower in the α w = 700 Qd(λλ) green wavelengths (Figure 4). Soybean had much ∫400 stronger absorption across all wavelengths and did not have such a large decrease in the green wavelengths. The αw for soybean differed by only 6% and the difference between green and red αw was only 9% for Arabidopsis (Table 1). Although there is a larger difference between the green and blue LEDs (23%), significant overlap in output spectra and the lower efficiency of blue light conversion to reducing power

3 decreased the effective difference in αw between these colors. The lack of significant differences in absorbed light for the different LED colors resulted in similar photosynthetic rates.

Table 1: The weighted absorption (αw) for the three colored LEDs singly and in concert. The weighted absorption is the integrated leaf absorption times the LED quantum output and then normalized to the LED quantum output.

LED ColorArabidopsis Soybean White 0.74 0.90 Red 0.72 0.90 Green 0.63 0.87 Blue 0.86 0.93 Conclusion The three colored LEDs’ output spectra are absorbed by plant species differently. Because of plants’ ability to capture much of the PAR spectrum and the band- width of the LED output, most of the light striking the leaf is absorbed and used in photosynthesis. When examining the photosynthetic response to light, it is important to normalize the light to the plant’s absorp- tion. When used in concert, the three LED colors give an apparent white light well suited for many photosyn- thesis studies. The 6400-18 RGB Light Source delivers white light† that is near solar irradiance levels without the heat generation associated with broad spectrum sources.

Acknowledgements Thanks to R. Ott at the University of Nebraska-Lincoln for providing the soybean seed stock.

Bibliography McCree, K.J. (1972) The Action Spectrum, Absorp- tance and Quantum Yield of photosynthesis in Crop Plants. Agricultural Meteorology 9: 191-216

Blankenship, R.E. (2002) Molecular Mechanisms of Photosynthesis. Blackwell Science Ltd., Oxford, United Kingdom.

®

4647 Superior Street • P.O. Box 4425 • Lincoln, Nebraska 68504 USA North America: 800-447-3576 • International: 402-467-3576 • FAX: 402-467-2819 • [email protected] • [email protected] • www.licor.com In Germany and Norway – LI-COR Biosciences GmbH: +49 (0) 6172 17 17 771 • [email protected] • [email protected] In UK and Ireland – LI-COR Biosciences UK Ltd.: +44 (0) 1223 422102 • [email protected] • [email protected] PPS-256 Printed in USA.